1. Field of the Invention
The present invention relates to an oil pressure circuit for an automatic transmission.
2. Description of Related Art
Automatic transmissions are designed such that a power transmission system of a multistage transmission of a planetary geartrain type is controlled, that is, its speed stages are shifted, by switching coupled or engaged states of transmission coupling or engaging friction elements such as clutches, brakes and so on.
The friction elements for shifting speed stages of an automatic transmission are operated by hydraulic actuators. Oil pressures are supplied or released to or from the hydraulic actuators through oil pressure circuits provided in the automatic transmission. The oil pressure circuits contains at least plural shift valves such as a 1-2 shift valve and a 2-3 shift valve, and the shift valves are shifted by pilot pressures. Recently, many automatic transmissions are designed such that shifting speed stages is controlled by electronically regulating the pilot pressure for the shift valves by means of solenoids. Japanese Patent Publication (Kokai) No. 92,351/1986 discloses a control system in which such a solenoid is provided each for the shift valves.
Japanese Patent Publication No. 18,780/1987 discloses another oil pressure control system of an automatic transmission in which solenoids are incorporated into the oil pressure control circuit system, enabling an electronic control of shifting speed stages. This system is such that a multistage transmission gear mechanism is provided with first and second coupling means (friction coupling elements) which provides four speed stages from first to fourth speed stages in accordance with a combination of engagement and disengagement of the first and second coupling means.
In this transmission gear mechanism, a first shift valve is provided to alternatively select a supply or a suspension of the supply of an operating oil for coupling the second coupling means and the first shift valve in turn is shifted by means of first pilot pressure generated by a first solenoid that is arranged to generate the first pilot pressure in such a manner that the second coupling means is brought into engagement at second, third and third speed stages and into disengagement at first speed stage. A supply or a suspension of the supply of an operating oil for coupling the first coupling means is shifted by a second shift valve. Shifting of the second shift valve is effected by second pilot pressure generated by a second solenoid which in turn generates the second pilot pressure so as to engage the first coupling means at third and fourth speed stages and to disengage it at first and second speed stages.
In this prior art transmission gear mechanism, a third solenoid is additionally provided which can generate a pilot pressure at third speed stage in a manner to prevail over the first pilot pressure and consequently act on the second coupling means for releasing the disengagement of the second coupling means. Accordingly, at the third speed stage, the disengagement of the second coupling means can be actually released even if the first solenoid would generate a pilot pressure which acts for engagement of the second coupling means.
In addition to the first and second coupling means, a third coupling means is further provided so as to permit a transmission of a torque in a reverse direction to the input shaft side from the output shaft side of the multistage transmission gear mechanism when the third coupling means is coupled or engaged, thereby ensuring engine braking. Although the third coupling means is designed so as to be engaged at any arbitrary speed stage, it is impossible to have the third coupling means engaged at all of the four speed stages because, if it were engaged at all of the first, second and third coupling means, the multistage transmission gear mechanism causes a so-called "internal lock", whereby no power can be transmitted. More specifically, the transmission gear mechanism as disclosed in this published Japanese patent application is designed such that the third coupling means is disengaged at fourth speed stage when the first and second coupling means are both engaged and it is engaged at the first, second, and third speed stages. In order to engage or disengage the third coupling means, it can be conceived that a fourth solenoid is additionally provided for controlling the third coupling means. However, such a fourth solenoid makes a structure of the oil pressure circuit more complex and expensive.
When the speed stage is shifted between the second and third speed stages where coupling the third coupling means is effected, a deviation in timings of switching the first and second coupling means gives rise to a temporary state of the fourth speed stage where an internal lock is caused, whereby a shock is likely to occur at the time of shifting the speed stages. More specifically, given an uncoupled state of the first coupling means and a coupled state of the second coupling means at the second speed stage as well as a coupled state of the first coupling means and an uncoupled state of the second coupling means at the third speed stage, on the one hand, if a timing of engaging or coupling the first coupling means would be earlier than a timing of releasing the coupling of or disengaging the second coupling means at the time of upshifting from the second to the third speed stages, there can be temporarily caused a situation in which the first and second coupling means are both in a coupled state, that is, in a state in which the speed stage is at fourth speed stage. Given the above, on the other hand, there can be likewise caused a situation in which a state of the fourth speed stage is temporarily caused if a timing of engaging the second coupling means would be earlier than a timing of disengaging the first coupling means at the time of downshifting from the third speed stage to the second speed stage.
The transmission gear mechanism having substantially the same apparatus structure as shown in FIG. 1 of this application is disclosed in pending U.S. Patent Application Ser. No. 926,840 filed Nov. 3, 1986 based on U.S. Ser. No. 665,044 filed Oct. 26, 1984 claiming Japanese Patent Application No. 202,042/1983 published as Publication (kokai) No. 95,236/1985. U.S. Pat. No. 4,665,774 issued May 19, 1987 based on U.S. Patent Application Ser. No. 746,071 filed June 18, 1985 claiming Japanese Patent Application No. 128,026/1984 published as Publication (kokai) No. 6,451/1986, pending U.S. Patent Application Ser. No. 31,612 filed Mar. 30, 1987 claiming Japanese Patent Application No. 75,697/1986 published as Publication (kokai) No. 233,547/1987, and pending U.S. Patent Application Ser. No. 32,611 filed Mar. 31, 1987 claiming Japanese Patent Application No. 77,090/1986 published as Publication (kokai) No. 233,551/1987, all of which are assigned to Mazda Motor Corporation.